Since the industrial revolution, the amount of CO2 in the atmosphere has risen from 280 ppm in 1800 to 370 ppm in 2000, mainly due to the consumption of fossil fuels. More than half of the energy used in the United States comes from the use of coal, and it is mostly used to generate electricity. Unfortunately, CO2 is one of the greenhouse gases considered to be responsible for global warming. Moreover, the increased atmospheric CO2 concentration will acidify the ocean and will change the chemistry of the surface ocean, leading to a potentially detrimental impact on the ecosystem. In order to meet the ever-increasing global energy demands, while stabilizing the atmospheric CO2 level, current carbon emissions should be significantly reduced.
There have been significant research and development activities in the area of carbon capture and storage (CCS), including a number of integrated technologies (e.g., chemical looping processes) to combine CO2 capture with electricity/chemical/fuel production. Chemical looping processes involve a sorbent, typically a metal, or more likely a low oxidation state metal oxide that can be oxidized in air. The oxide is reduced by carbonaceous fuels in a subsequent step. A variation of this approach oxidizes the metal not in air but in a chemical reaction with steam to produce a pure stream of H2. The chemical looping processes also allow the inherent generation of the sequestration-ready CO2 stream at higher pressures.
Once captured, CO2 can be stored via geological sequestration, ocean disposal, mineral carbonation, and biological fixation. The mineral sequestration scheme is particularly attractive, since this process converts CO2 into thermodynamically stable carbonates via the reaction of CO2 with widely available non-carbonate minerals, such as serpentine and olivine. Therefore, the mineral sequestration process eliminates the risk of accidental CO2 releases. The reaction underlining mineral carbonation mimics natural chemical transformations of CO2, such as the weathering of rocks. The main challenges of this storage method have been the slow dissolution kinetics and large energy requirement associated with the mineral processing.